1. Field of the Invention
This invention relates to a hydraulic pressure control device in an automatic transmission, and more particularly to improvements in a hydraulic pressure control device in an automatic transmission provided thereon with a mechanism capable of regulating control oil pressure in response to a command from calculating means, the hydraulic pressure control device being suitable for use in an electronic automatic transmission for a motor vehicle.
2. Related Art
There has been widely known an automatic transmission for a motor vehicle, wherein a gear transmission mechanism and a plurality of frictionally engaging devices are provided, and a hydraulic pressure control device is operated to selectively change over the engagements of the frictionally engaging devices, to thereby achieve any one of a plurality of gear stages.
In general, each frictionally engaging device is comprised of two sets of friction disc elements which are relatively movably supported, and a hydraulic pressure servo device for driving the friction disc elements. When oil pressure is supplied to the hydraulic pressure servo device, the two sets of friction disc elements are strongly urged against each other, whereby the two sets of friction disc elements can be connected to each other in a manner to transmit torque therebetween.
The above-described basic oil pressure to the frictionally engaging devices is referred to generally as line pressure or line oil pressure. This line oil pressure has heretofore been regulated by introducing throttle oil pressure, variable as commensurate to a throttle opening (representing an engine load), to a control port of a primary regulator valve for controlling the line oil pressure. Furthermore, in general, this throttle oil pressure has heretofore been generated by a throttle valve, in which the resilient force of a spring, increased commensurately to a depression value of an accelerator pedal, is rendered to a spool thereof.
In recent years, electronic automatic transmissions have been developed wherein an electronic circuit forms a main portion of a control circuit. In such control circuits, information on the throttle opening has been processed in the form of an electronic signal. Specifically, a device has been developed such that the line oil pressure is controlled in response to an electronic signal relating to the throttle opening (For example, Japanese Utility Model Kokai (Laid-Open) No. 125555/1981).
With the line oil pressure or the throttle oil pressure controlled in response to a command value from a computer (calculating means) as described above, precise control has become possible. For example, during non-shift running the line oil pressure is controlled and regulated as low as possible commensurate to an engine load, a vehicle speed and the like, with a suitable margin of safety being provided, so that wasteful power loss in an oil pump can be avoided.
However, when the oil pressure in the hydraulic pressure control device is controlled in response to a command value from the computer in accordance with signals inputted from various sensors as described above, even as accurately as the electronic system functions, a problem still remains that control as precise as is desirable is not possibly carried out. Since the subject to be controlled is an oil pressure system (a mechanical system), variations peculiar to the machine system such as dimensional variations of the pressure regulating valve, variations in the spring constant of a return spring and variations in use conditions such as oil temperature, cannot be avoided.